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Semiconductor-based experiments for neutrinoless double beta decay search
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Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 141–145 www.elsevier.com/locate/npbps
Semiconductor-based experiments for neutrinoless double beta decay search Marik Barnab´e Heider for the GERDA collaboration∗ Max-Planck-Institut f¨ur Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
Abstract Three experiments are employing semiconductor detectors in the search for neutrinoless double beta (0νββ) decay: COBRA, Majorana and GERDA. COBRA is studying the prospects of using CdZnTe detectors in terms of achievable energy resolution and background suppression. These detectors contain several ββ emitters and the most promising for 0νββ-decay search is 116 Cd. Majorana and GERDA will use isotopically enriched high purity Ge detectors to search for 0νββ-decay of 76 Ge. Their aim is to achieve a background ≤ 10−3 counts/(kg·y·keV) at the Qββ -value, a 100-fold improvement compared to the present state-of-art. Majorana will operate Ge detectors in electroformed-Cu vacuum cryostats. A first cryostat housing a natural-Ge detector array is currently under preparation. In contrast, GERDA is operating bare Ge detectors submerged in liquid argon. The construction of the GERDA experiment is completed and a commissioning run started in June 2010. A string of natural-Ge detectors is operated to test the complete experimental setup and to determine the background before submerging the detectors enriched in 76 Ge. An overview and a comparison of these three experiments will be presented together with the latest results and developments. Keywords: Semiconductor detector, Neutrinoless double beta decay, Radiation detector, CZT detector, Ge detector
1. Introduction Semiconductor technology is one of the basic tools in fundamental physics. A wide range of semiconductor detectors is applied to particle detection in many areas of physics. Three experiments use semiconductors to search for neutrinoless double beta (0νββ) decay. The 0νββ-decay is the only known experimental possibility to test the Majorana nature of neutrinos. If it is detected it will also provide information on the Majorana neutrino mass. The COBRA experiment uses CdZnTe (CZT) detectors, Majorana and GERDA use Ge detectors. Semiconductor detectors present several advantages for 0νββ-decay search. The detector-grade semiconductors are high-purity materials which is an obvious benefit for low background experiments. For all three experiments, the detectors are made of the ββ∗ Email:
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0920-5632/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2012.09.023
decay source material, which has a high density. Consequently the detection efficiency is very good. Another advantage is the excellent energy resolution of the detectors (∼2-3 keV for Ge and ∼15-20 keV for CZT). As the 0νββ-decay mode can be distinguished from the much more probable 2ν mode only by the difference in energy lost in the detector, energy resolution is important. All three experiments propose a modular design allowing background rejection by anti-coincidence between the detectors. A second benefit of such a design is the possibility of an easy upgrade to a larger source material mass. The CZT detectors can be operated at room temperature which is advantageous compared to Ge detectors which have to be operated at cryogenic temperature to avoid electron-hole pair creation by thermal excitation. An important benefit of using semiconductor detector technology is the availability of a wide established industrial base on which the experiments can rely for support.
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2. The COBRA experiment
The isotopic composition of CZT semiconductor detectors used by COBRA [1] contains nine ββ emitters (including 2β+ , ECβ+ and ECEC processes). From these isotopes, the most promising for investigating the 0ν mode is 116 Cd. The Qββ -value of this isotope is 2809 keV which is beyond most gamma lines produced by the natural decay chains of U and Th. Before building a large-scale experiment, the feasibility of operating CZT detectors for the search of 0νββ-decay in terms of achievable energy resolution and background suppression has to be investigated. The CZT detectors manufactured by present-day crystal growth technology typically do not exceed a mass of the order of 10 g. Since a much larger source material mass is required for 0νββ-decay search, the crystals must be arranged in a large array. After operating single detectors and a small array of four detectors, an array of sixteen 1 cm3 (6.5 g) coplanar-grid CZT detectors (figure 1) has been installed in 2006 and operated deep underground at the Laboratori Nazionali del Gran Sasso (LNGS), Italy. The 4x4 detector array (total mass 103.9 g) was mounted in an inner Cu shield surrounded by a lead castle inside a Faraday cage and a neutron shield. The energy resolution (FWHM) of the detectors varied between 3.5% and 8.5% at 2.8 MeV. A wide room for improvement is available before reaching the state-ofart energy resolution of CZT detector technology. An exposure of 18 kg·days was accumulated during the operation of this setup until the end of 2008. The COBRA collaboration obtained physics results on the 4fold forbidden 113 Cd β-decay as well as 0νββ-decay limits for several isotopes [2]. The setup will be upgraded in a near future with an array of 64 detectors (about 0.5 kg total mass). To reach background levels necessary to significantly improve 0νββ-decay rate limits, orders of magnitude improvement are needed. As a promising track towards reducing background, COBRA carries out R&D on detector pixelisation. In one of the pursued approaches highly pixelised detectors combined with the use of pulse shape analysis provide tracking information in a manner analogous to a solid-state version of a time-projection chamber. This technique enables particle identification and thus rejection of alpha, muon and gamma ray backgrounds. In another approach, the surface events are rejected by applying a fiducial cut excluding the edge pixels. A 36 g CZT detector of the latter design was running at LNGS from September 2009 to January 2010 giving promising results.
Figure 1: The COBRA experiment at the Laboratori Nazionali del Gran Sasso. Left: A single coplanar-grid CZT detector. Right: Setup for operating a 64 detector array.
3. Germanium detector experiments Both Majorana [3] and GERDA [4] will search for 0νββ-decay of 76 Ge using germanium spectrometry, currently the leading technique in the search of 0νββdecay. The most sensitive experiments to date have been Heidelberg-Moscow (HdM) [5] and the International Germanium Experiment (IGEX) [6], both of which used conventional p-type Ge detectors operated in vacuum cryostats. The detectors were isotopically enriched to 86% concentration of 76 Ge. This isotope has a Qββ value of 2039 keV which lies below the energy of the radiation from some natural background sources. Most notable environmental backgrounds are the isotopes 208 Tl and 214 Bi from the primordial decay chains of 232 Th and 238 U, respectively. The germanium itself can be activated by spallation from cosmic rays producing a variety of radioactive isotopes. From these, 68 Ge and 60 Co are important contributors to the background at Qββ . The design of the experiments is therefore focused on shielding and minimising the environmental radioactive backgrounds and the exposure of the Ge detectors above ground must be limited to prevent cosmogenic activation. Majorana and GERDA aim for a 100-fold reduction in background around Qββ compared to their predecessor experiments HdM and IGEX, which achieved 10−1 counts/(kg·y·keV). This ambition requires innovative design approaches and strict selection of lowradioactivity materials. Moreover, novel techniques for active background suppression are necessary. Advanced signal recognition techniques are pursued allowing background suppression beyond what is achievable by passive shielding and avoiding background sources. The techniques use by Majorana and GERDA include pulse shape analysis [7] and detector anti-coincidence. In addition, R&D is performed on detector segmenta-
M. Barnabé Heider / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 141–145
tion [8] and liquid argon (LAr) scintillation acting as anti-coincidence veto [9]. Several Ge-detector technologies capable of advanced signal recognition were studied in the frame of the Majorana and GERDA projects (highly segmented Ge detector, point contact Ge detector, Broad-Energy Ge detector). The primary detector configurations chosen for custom made detectors for both Majorana and GERDA are modified versions of the commercially available ptype Broad-Energy Ge (BEGe) detector (∼800 g). Such detectors have several benefits resulting from the small size of their p+ electrode. The most important advantage for 0νββ-decay search is the enhanced capability for background suppression by distinguishing the interactions of electrons from beta-decays from the interactions of multiple-scattered photons inside the detector via pulse-shape analysis. Another consequence of the small p+ electrode is its low capacitance, resulting in lower noise and thus superior energy resolution (as low as 1.6 keV FWHM at 1.33 MeV) and lower energy threshold of BEGe detectors. The approach to the passive shielding against external radioactivity represents the fundamental difference between Majorana and GERDA. Majorana will operate arrays of Ge detectors inside high purity electroformedCu vacuum cryostats with a low-background Cu and lead shield surrounding the setup. GERDA will operate an array of bare Ge detectors submersed in LAr, which serves simultaneously as cooling medium and as a shield against external radiation, with an external water shield. There is an open exchange of knowledge and technologies between the two collaborations and they have the intention to merge for a ton scale experiment with the goal to achieve a background on the order of 10−4 cts/(kg·y·keV) in the Qββ region. The experiments will measure (or derive a lower limit for) the half-life of the 0νββ-decay process. The resulting on the effective electron neutrino mass sensitivity mββ depends on the theoretical value for the nuclear matrix element and the kinematical phase space factor [10]. While the initial phases of Majorana and GERDA should cover the quasi-degenerate neutrino mass region (upper limit of mββ = 0.09 − 0.15 eV) a ton-scale ex periment could be sensitive to mββ ∼ 10 meV, which corresponds to the inverted ordering scenario.
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Figure 2: The Majorana Demonstrator at DUSEL underground laboratory, in Homestake mine, USA: electroformed-Cu vacuum cryostats housing Broad-Energy Ge detectors. The arrays are placed in a Cu and lead shield.
detectors and 30 kg of enriched-Ge detectors will be installed in multi-crystal high-purity vacuum cryostats (figure 2). In addition to the compact low-background Cu and lead shield, an active muon veto will be used. The expected background in the Qββ region is < 10−3 counts/(kg·y·keV). The experiment will be running for 3 years and should accumulate an exposure of 90 kg·y translating into a 76 Ge 0νββ-decay half life of T1/2 ≥ 1026 years (90% CL). The objective is to demonstrate a background low enough to justify building a ton scale Ge-experiment. The low energy threshold and low noise of the BEGe detectors are especially useful for the second goal of Majorana, which is the ability to search for dark matter by direct detection. The collaboration dedicates a lot of effort to the development of ultra-low noise signal readout electronics made of radio-pure material. To minimize the contribution of signal cables to the electronic noise the read-out electronics will be mounted next to the detectors. The electroforming of Cu for the detector vacuum cryostats, which is done deep underground at DUSEL to prevent cosmogenic activation, will start this year. The first cryostat with 20 kg of natural-BEGe detectors (18 detectors mounted in strings of 4 or 6) is expected to be ready for data-taking in fall 2011.
3.1. The Majorana Project Majorana will be located deep underground at DUSEL in Homestake mine, South Dakota (4000-7000 m w.e. below surface). In the initial phase of Majorana, called the Demonstrator, 30 kg of natural-Ge
3.2. The GERDA experiment GERDA is installed in Hall A of the Laboratori Nazionali del Gran Sasso (3800 m w.e.). The core feature of GERDA is its array of enriched-Ge detectors
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Figure 3: The GERDA experiment in Hall A of the Laboratori Nazionali del Gran Sasso, Italy. Left: GERDA cryostat surrounded by a water tank and topped by a cleanroom with a lock system. Right: A Ge detector string operated inside the liquid argon cryostat.
operated bare in the cryogenic liquid in a modular arrangement of strings. To minimize the amount of radioactive contamination close to the detectors their support structure and electrical connections are made of selected ultra-pure materials (low-background Cu, silicon and PTFE) with minimal mass. The cryostat, produced from selected low-background austenitic stainless steel [11], is filled with 64 m3 of LAr. It has an additional internal Cu layer to shield the array from the residual background of the stainless steel wall. A lock system allows to insert and remove the detectors without contaminating the LAr vessel. The detector handling is performed in a clean environment, under nitrogen atmosphere in a glove box inside a clean room on top of the structure. The cryostat is contained in a tank filled with 580 m3 of ultra-pure water. The water tank is equipped with 66 PMTs and acts as a muon veto. The water tank is designed to reduce the external gamma, neutron and muon background to 10−4 counts/(kg·y·keV). Figure 3 shows the GERDA experimental setup. In the first phase of GERDA, reprocessed enriched diodes from the HdM and IGEX experiments will be deployed. In total, 8 detectors (total mass ∼18 kg) enriched in 76 Ge to 86% will be operated. In addition, 6 reprocessed low-background natural-Ge detectors from Genius-TF [12] (total mass ∼16 kg) are available. The intention is to reach a background around Qββ of less than 10−2 cts/(kg·y·keV). Assuming an exposure of ∼15 kg·y a half life limit for 76 Ge 0νββ-decay
of T1/2 > 3.0 · 1025 y (90% C.L.) is expected. Therefore, the Phase-I sensitivity should allow a statistically significant statement regarding the claim of detection [13]. In GERDA Phase-II, new enriched-Ge detectors will be added (increasing the total mass to ∼40 kg) and one order of magnitude improvement will be necessary to reach a background < 10−3 counts/(kg·y·keV)) around Qββ . With an exposure of 100 kg·y the half life limit should be improved to T1/2 > 2 · 1026 y, similarly as the Majorana Demonstrator. The construction of the experiment was completed with the installation of the main components (cryostat, water tank, clean room, muon veto, one-string lock) at LNGS between 2008 and 2010. Both the LAr cryostat and the water tank were filled. The GERDA collaboration has five years of experience with bare detector operation in cryogenic liquids. The detector handling procedure has been well tested and the long term stability in LAr is established. The Phase-I detectors were reprocessed at Canberra Semiconductor Olen [14] for a common design with new contacts optimized for LAr. All detectors are mounted in low-mass holders and were tested in LAr. An energy resolution of 2.5 keV (FWHM) at 1.33 MeV was obtained. After deployments of detector mock-ups in the cryostat to test the front-end electronics and the mechanical system, a commissioning run with a string of three natural-Ge detectors started in June 2010. The purpose of the commissioning run is to measure and understand the background in the GERDA cryostat, and to test the system before submerging the enriched detectors. Presently the energy resolution achieved during the calibration runs is ∼4-5 keV (FWHM) for the 2.6 MeV gamma line of 228 Th. A dedicated campaign will be performed to improve the spectroscopic performance of the detectors. In 2006 37.5 kg of germanium enriched in 76 Ge at 86% was acquired for the Phase-II detectors. The Ge was recently reduced to metallic form and purified by zone refining with a yield of 97% for 6N grade material. The exposure to cosmic rays was minimised by underground storage in between the process steps. The whole production chain from the acquisition of isotopically modified material to the production and testing of four working BEGe detectors was verified with depleted germanium left over after the 76 Ge enrichment process. The crystal pulling for the first batches of Phase-II detectors will be done at Canberra Oakridge [15] while R&D is also performed at Institut f¨ur Kristallzuechtung, Berlin [16].
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4. Summary Semiconductor detector is one of the leading technologies for particle detection and identification. COBRA, Majorana and GERDA use semiconductor detectors with the ultimate aim to detect 0νββ-decay. COBRA uses CZT detectors with the most promising 0νββdecaying isotope being 116 Cd. A campaing with a 16 detector setup was completed at LNGS and the next stage is the installation of an array with 64 crystals. Further R&D is required to prove the validity of the COBRA technique, mainly to reduce their background to the required sensitivity and to improve the energy resolution of the detectors. Pixellized CZT detectors are potentially a powerfull tool for background reduction. Both Majorana and GERDA will search for 0νββdecay of 76 Ge. Majorana will operate Ge detectors in electroformed-Cu vacuum cryostats while GERDA will deploy an array of bare Ge detectors in LAr. The Majorana collaboration has acquired their first natural-Ge detectors for the Demonstrator phase. The electroforming of the Cu will start this year underground. The first cryostat should be ready for data taking in 2011. The construction of the GERDA experiment was completed at LNGS in early 2010. Since June 2010, a commisionning run with a string of three natural-Ge detectors is under way. The objective is to verify the background level as well as the complete experimental setup before submerging the enriched detectors. The Phase-I enriched detectors themselves were tested and are ready for deployment. References [1] K. Zuber, Phys. Lett. B 519 (2001) 1-7. [2] J.V. Dawson et al., Nucl. Phys. A 818 (2009) 264-278. J.V. Dawson et al., Phys. Rev. C 80 (2009) 025502. [3] S.R. Elliott, for the Majorana collaboration, [arXiv:nuclex/0807.1741v1] (2008). [4] I. Abt et al. (GERDA collaboration), [arXiv:hep-ex/0404039] (2004). S. Sch¨onert for the GERDA collaboration, Nucl. Phys. B (Proc. Suppl.) 145 (2005) 242-245. K.T. Kn¨opfle for the GERDA collaboration, [arXiv:hepex/0809.5207v1] (2008). [5] A. Balysh et al., Phys. Rev. D 55 (1997) 54. [6] C. E. Aalseth et al. (IGEX Collaboration), Phys. of Atomic Nuclei 63 (2000) 1225. [7] D. Budj´asˇ, M. Barnab´e Heider, O. Chkvorets, N. Khanbekov and S. Sch¨onert, JINST 4 P10007 (2009), doi:10.1088/17480221/4/10/P10007. [8] S.R. Elliott et al., Nucl. Instrum. Meth. A 558 (2006) 504. I. Abt, A. Caldwell, K. Kr¨oninger, J. Liu, X. Liu, B. Majorovits, Nucl. Instrum. Meth. A 583 (2007) 332-340. [9] M. Di Marco, P. Peiffer, S. Sch¨onert, Nucl. Phys. Proc. Suppl. 172 (2007) 45-48.
[10] [11] [12] [13] [14] [15] [16]
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P. Peiffer, T. Pollmann, S. Sch¨onert, A. Smolnikov and S. Vasiliev, JINST 3 P08007 (2008). A. Smolnikov, P. Grabmayr, Phys. Rev. C 81 (2010) 028502. W. Maneschg et al., Nucl. Instrum. Meth. A 593 (2008) 448453. Baudis L et al., [arXiv:hep-ex/0012022v1] (2000). H.V. Klapdor-Kleingrothaus, I.V. Krivosheina, and O. Chkvorets, Phys. Lett. B 586 (2004) 198-212. Canberra Semiconductor NV, Lammerdries 25, B-2430 Olen, Belgium. Canberra Industries, Inc, Tennelec division, 107 Union Valley Road, Oak Ridge Tennessee, USA. Leibniz-Institut f¨ur Kristallz¨uchtung, Max-Born-Str.2 D-12489 Berlin, Germany.
Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 141–145 www.elsevier.com/locate/npbps
Semiconductor-based experiments for neutrinoless double beta decay search Marik Barnab´e Heider for the GERDA collaboration∗ Max-Planck-Institut f¨ur Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
Abstract Three experiments are employing semiconductor detectors in the search for neutrinoless double beta (0νββ) decay: COBRA, Majorana and GERDA. COBRA is studying the prospects of using CdZnTe detectors in terms of achievable energy resolution and background suppression. These detectors contain several ββ emitters and the most promising for 0νββ-decay search is 116 Cd. Majorana and GERDA will use isotopically enriched high purity Ge detectors to search for 0νββ-decay of 76 Ge. Their aim is to achieve a background ≤ 10−3 counts/(kg·y·keV) at the Qββ -value, a 100-fold improvement compared to the present state-of-art. Majorana will operate Ge detectors in electroformed-Cu vacuum cryostats. A first cryostat housing a natural-Ge detector array is currently under preparation. In contrast, GERDA is operating bare Ge detectors submerged in liquid argon. The construction of the GERDA experiment is completed and a commissioning run started in June 2010. A string of natural-Ge detectors is operated to test the complete experimental setup and to determine the background before submerging the detectors enriched in 76 Ge. An overview and a comparison of these three experiments will be presented together with the latest results and developments. Keywords: Semiconductor detector, Neutrinoless double beta decay, Radiation detector, CZT detector, Ge detector
1. Introduction Semiconductor technology is one of the basic tools in fundamental physics. A wide range of semiconductor detectors is applied to particle detection in many areas of physics. Three experiments use semiconductors to search for neutrinoless double beta (0νββ) decay. The 0νββ-decay is the only known experimental possibility to test the Majorana nature of neutrinos. If it is detected it will also provide information on the Majorana neutrino mass. The COBRA experiment uses CdZnTe (CZT) detectors, Majorana and GERDA use Ge detectors. Semiconductor detectors present several advantages for 0νββ-decay search. The detector-grade semiconductors are high-purity materials which is an obvious benefit for low background experiments. For all three experiments, the detectors are made of the ββ∗ Email:
[email protected]
0920-5632/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2012.09.023
decay source material, which has a high density. Consequently the detection efficiency is very good. Another advantage is the excellent energy resolution of the detectors (∼2-3 keV for Ge and ∼15-20 keV for CZT). As the 0νββ-decay mode can be distinguished from the much more probable 2ν mode only by the difference in energy lost in the detector, energy resolution is important. All three experiments propose a modular design allowing background rejection by anti-coincidence between the detectors. A second benefit of such a design is the possibility of an easy upgrade to a larger source material mass. The CZT detectors can be operated at room temperature which is advantageous compared to Ge detectors which have to be operated at cryogenic temperature to avoid electron-hole pair creation by thermal excitation. An important benefit of using semiconductor detector technology is the availability of a wide established industrial base on which the experiments can rely for support.
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2. The COBRA experiment
The isotopic composition of CZT semiconductor detectors used by COBRA [1] contains nine ββ emitters (including 2β+ , ECβ+ and ECEC processes). From these isotopes, the most promising for investigating the 0ν mode is 116 Cd. The Qββ -value of this isotope is 2809 keV which is beyond most gamma lines produced by the natural decay chains of U and Th. Before building a large-scale experiment, the feasibility of operating CZT detectors for the search of 0νββ-decay in terms of achievable energy resolution and background suppression has to be investigated. The CZT detectors manufactured by present-day crystal growth technology typically do not exceed a mass of the order of 10 g. Since a much larger source material mass is required for 0νββ-decay search, the crystals must be arranged in a large array. After operating single detectors and a small array of four detectors, an array of sixteen 1 cm3 (6.5 g) coplanar-grid CZT detectors (figure 1) has been installed in 2006 and operated deep underground at the Laboratori Nazionali del Gran Sasso (LNGS), Italy. The 4x4 detector array (total mass 103.9 g) was mounted in an inner Cu shield surrounded by a lead castle inside a Faraday cage and a neutron shield. The energy resolution (FWHM) of the detectors varied between 3.5% and 8.5% at 2.8 MeV. A wide room for improvement is available before reaching the state-ofart energy resolution of CZT detector technology. An exposure of 18 kg·days was accumulated during the operation of this setup until the end of 2008. The COBRA collaboration obtained physics results on the 4fold forbidden 113 Cd β-decay as well as 0νββ-decay limits for several isotopes [2]. The setup will be upgraded in a near future with an array of 64 detectors (about 0.5 kg total mass). To reach background levels necessary to significantly improve 0νββ-decay rate limits, orders of magnitude improvement are needed. As a promising track towards reducing background, COBRA carries out R&D on detector pixelisation. In one of the pursued approaches highly pixelised detectors combined with the use of pulse shape analysis provide tracking information in a manner analogous to a solid-state version of a time-projection chamber. This technique enables particle identification and thus rejection of alpha, muon and gamma ray backgrounds. In another approach, the surface events are rejected by applying a fiducial cut excluding the edge pixels. A 36 g CZT detector of the latter design was running at LNGS from September 2009 to January 2010 giving promising results.
Figure 1: The COBRA experiment at the Laboratori Nazionali del Gran Sasso. Left: A single coplanar-grid CZT detector. Right: Setup for operating a 64 detector array.
3. Germanium detector experiments Both Majorana [3] and GERDA [4] will search for 0νββ-decay of 76 Ge using germanium spectrometry, currently the leading technique in the search of 0νββdecay. The most sensitive experiments to date have been Heidelberg-Moscow (HdM) [5] and the International Germanium Experiment (IGEX) [6], both of which used conventional p-type Ge detectors operated in vacuum cryostats. The detectors were isotopically enriched to 86% concentration of 76 Ge. This isotope has a Qββ value of 2039 keV which lies below the energy of the radiation from some natural background sources. Most notable environmental backgrounds are the isotopes 208 Tl and 214 Bi from the primordial decay chains of 232 Th and 238 U, respectively. The germanium itself can be activated by spallation from cosmic rays producing a variety of radioactive isotopes. From these, 68 Ge and 60 Co are important contributors to the background at Qββ . The design of the experiments is therefore focused on shielding and minimising the environmental radioactive backgrounds and the exposure of the Ge detectors above ground must be limited to prevent cosmogenic activation. Majorana and GERDA aim for a 100-fold reduction in background around Qββ compared to their predecessor experiments HdM and IGEX, which achieved 10−1 counts/(kg·y·keV). This ambition requires innovative design approaches and strict selection of lowradioactivity materials. Moreover, novel techniques for active background suppression are necessary. Advanced signal recognition techniques are pursued allowing background suppression beyond what is achievable by passive shielding and avoiding background sources. The techniques use by Majorana and GERDA include pulse shape analysis [7] and detector anti-coincidence. In addition, R&D is performed on detector segmenta-
M. Barnabé Heider / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 141–145
tion [8] and liquid argon (LAr) scintillation acting as anti-coincidence veto [9]. Several Ge-detector technologies capable of advanced signal recognition were studied in the frame of the Majorana and GERDA projects (highly segmented Ge detector, point contact Ge detector, Broad-Energy Ge detector). The primary detector configurations chosen for custom made detectors for both Majorana and GERDA are modified versions of the commercially available ptype Broad-Energy Ge (BEGe) detector (∼800 g). Such detectors have several benefits resulting from the small size of their p+ electrode. The most important advantage for 0νββ-decay search is the enhanced capability for background suppression by distinguishing the interactions of electrons from beta-decays from the interactions of multiple-scattered photons inside the detector via pulse-shape analysis. Another consequence of the small p+ electrode is its low capacitance, resulting in lower noise and thus superior energy resolution (as low as 1.6 keV FWHM at 1.33 MeV) and lower energy threshold of BEGe detectors. The approach to the passive shielding against external radioactivity represents the fundamental difference between Majorana and GERDA. Majorana will operate arrays of Ge detectors inside high purity electroformedCu vacuum cryostats with a low-background Cu and lead shield surrounding the setup. GERDA will operate an array of bare Ge detectors submersed in LAr, which serves simultaneously as cooling medium and as a shield against external radiation, with an external water shield. There is an open exchange of knowledge and technologies between the two collaborations and they have the intention to merge for a ton scale experiment with the goal to achieve a background on the order of 10−4 cts/(kg·y·keV) in the Qββ region. The experiments will measure (or derive a lower limit for) the half-life of the 0νββ-decay process. The resulting on the effective electron neutrino mass sensitivity mββ depends on the theoretical value for the nuclear matrix element and the kinematical phase space factor [10]. While the initial phases of Majorana and GERDA should cover the quasi-degenerate neutrino mass region (upper limit of mββ = 0.09 − 0.15 eV) a ton-scale ex periment could be sensitive to mββ ∼ 10 meV, which corresponds to the inverted ordering scenario.
143
Figure 2: The Majorana Demonstrator at DUSEL underground laboratory, in Homestake mine, USA: electroformed-Cu vacuum cryostats housing Broad-Energy Ge detectors. The arrays are placed in a Cu and lead shield.
detectors and 30 kg of enriched-Ge detectors will be installed in multi-crystal high-purity vacuum cryostats (figure 2). In addition to the compact low-background Cu and lead shield, an active muon veto will be used. The expected background in the Qββ region is < 10−3 counts/(kg·y·keV). The experiment will be running for 3 years and should accumulate an exposure of 90 kg·y translating into a 76 Ge 0νββ-decay half life of T1/2 ≥ 1026 years (90% CL). The objective is to demonstrate a background low enough to justify building a ton scale Ge-experiment. The low energy threshold and low noise of the BEGe detectors are especially useful for the second goal of Majorana, which is the ability to search for dark matter by direct detection. The collaboration dedicates a lot of effort to the development of ultra-low noise signal readout electronics made of radio-pure material. To minimize the contribution of signal cables to the electronic noise the read-out electronics will be mounted next to the detectors. The electroforming of Cu for the detector vacuum cryostats, which is done deep underground at DUSEL to prevent cosmogenic activation, will start this year. The first cryostat with 20 kg of natural-BEGe detectors (18 detectors mounted in strings of 4 or 6) is expected to be ready for data-taking in fall 2011.
3.1. The Majorana Project Majorana will be located deep underground at DUSEL in Homestake mine, South Dakota (4000-7000 m w.e. below surface). In the initial phase of Majorana, called the Demonstrator, 30 kg of natural-Ge
3.2. The GERDA experiment GERDA is installed in Hall A of the Laboratori Nazionali del Gran Sasso (3800 m w.e.). The core feature of GERDA is its array of enriched-Ge detectors
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Figure 3: The GERDA experiment in Hall A of the Laboratori Nazionali del Gran Sasso, Italy. Left: GERDA cryostat surrounded by a water tank and topped by a cleanroom with a lock system. Right: A Ge detector string operated inside the liquid argon cryostat.
operated bare in the cryogenic liquid in a modular arrangement of strings. To minimize the amount of radioactive contamination close to the detectors their support structure and electrical connections are made of selected ultra-pure materials (low-background Cu, silicon and PTFE) with minimal mass. The cryostat, produced from selected low-background austenitic stainless steel [11], is filled with 64 m3 of LAr. It has an additional internal Cu layer to shield the array from the residual background of the stainless steel wall. A lock system allows to insert and remove the detectors without contaminating the LAr vessel. The detector handling is performed in a clean environment, under nitrogen atmosphere in a glove box inside a clean room on top of the structure. The cryostat is contained in a tank filled with 580 m3 of ultra-pure water. The water tank is equipped with 66 PMTs and acts as a muon veto. The water tank is designed to reduce the external gamma, neutron and muon background to 10−4 counts/(kg·y·keV). Figure 3 shows the GERDA experimental setup. In the first phase of GERDA, reprocessed enriched diodes from the HdM and IGEX experiments will be deployed. In total, 8 detectors (total mass ∼18 kg) enriched in 76 Ge to 86% will be operated. In addition, 6 reprocessed low-background natural-Ge detectors from Genius-TF [12] (total mass ∼16 kg) are available. The intention is to reach a background around Qββ of less than 10−2 cts/(kg·y·keV). Assuming an exposure of ∼15 kg·y a half life limit for 76 Ge 0νββ-decay
of T1/2 > 3.0 · 1025 y (90% C.L.) is expected. Therefore, the Phase-I sensitivity should allow a statistically significant statement regarding the claim of detection [13]. In GERDA Phase-II, new enriched-Ge detectors will be added (increasing the total mass to ∼40 kg) and one order of magnitude improvement will be necessary to reach a background < 10−3 counts/(kg·y·keV)) around Qββ . With an exposure of 100 kg·y the half life limit should be improved to T1/2 > 2 · 1026 y, similarly as the Majorana Demonstrator. The construction of the experiment was completed with the installation of the main components (cryostat, water tank, clean room, muon veto, one-string lock) at LNGS between 2008 and 2010. Both the LAr cryostat and the water tank were filled. The GERDA collaboration has five years of experience with bare detector operation in cryogenic liquids. The detector handling procedure has been well tested and the long term stability in LAr is established. The Phase-I detectors were reprocessed at Canberra Semiconductor Olen [14] for a common design with new contacts optimized for LAr. All detectors are mounted in low-mass holders and were tested in LAr. An energy resolution of 2.5 keV (FWHM) at 1.33 MeV was obtained. After deployments of detector mock-ups in the cryostat to test the front-end electronics and the mechanical system, a commissioning run with a string of three natural-Ge detectors started in June 2010. The purpose of the commissioning run is to measure and understand the background in the GERDA cryostat, and to test the system before submerging the enriched detectors. Presently the energy resolution achieved during the calibration runs is ∼4-5 keV (FWHM) for the 2.6 MeV gamma line of 228 Th. A dedicated campaign will be performed to improve the spectroscopic performance of the detectors. In 2006 37.5 kg of germanium enriched in 76 Ge at 86% was acquired for the Phase-II detectors. The Ge was recently reduced to metallic form and purified by zone refining with a yield of 97% for 6N grade material. The exposure to cosmic rays was minimised by underground storage in between the process steps. The whole production chain from the acquisition of isotopically modified material to the production and testing of four working BEGe detectors was verified with depleted germanium left over after the 76 Ge enrichment process. The crystal pulling for the first batches of Phase-II detectors will be done at Canberra Oakridge [15] while R&D is also performed at Institut f¨ur Kristallzuechtung, Berlin [16].
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4. Summary Semiconductor detector is one of the leading technologies for particle detection and identification. COBRA, Majorana and GERDA use semiconductor detectors with the ultimate aim to detect 0νββ-decay. COBRA uses CZT detectors with the most promising 0νββdecaying isotope being 116 Cd. A campaing with a 16 detector setup was completed at LNGS and the next stage is the installation of an array with 64 crystals. Further R&D is required to prove the validity of the COBRA technique, mainly to reduce their background to the required sensitivity and to improve the energy resolution of the detectors. Pixellized CZT detectors are potentially a powerfull tool for background reduction. Both Majorana and GERDA will search for 0νββdecay of 76 Ge. Majorana will operate Ge detectors in electroformed-Cu vacuum cryostats while GERDA will deploy an array of bare Ge detectors in LAr. The Majorana collaboration has acquired their first natural-Ge detectors for the Demonstrator phase. The electroforming of the Cu will start this year underground. The first cryostat should be ready for data taking in 2011. The construction of the GERDA experiment was completed at LNGS in early 2010. Since June 2010, a commisionning run with a string of three natural-Ge detectors is under way. The objective is to verify the background level as well as the complete experimental setup before submerging the enriched detectors. The Phase-I enriched detectors themselves were tested and are ready for deployment. References [1] K. Zuber, Phys. Lett. B 519 (2001) 1-7. [2] J.V. Dawson et al., Nucl. Phys. A 818 (2009) 264-278. J.V. Dawson et al., Phys. Rev. C 80 (2009) 025502. [3] S.R. Elliott, for the Majorana collaboration, [arXiv:nuclex/0807.1741v1] (2008). [4] I. Abt et al. (GERDA collaboration), [arXiv:hep-ex/0404039] (2004). S. Sch¨onert for the GERDA collaboration, Nucl. Phys. B (Proc. Suppl.) 145 (2005) 242-245. K.T. Kn¨opfle for the GERDA collaboration, [arXiv:hepex/0809.5207v1] (2008). [5] A. Balysh et al., Phys. Rev. D 55 (1997) 54. [6] C. E. Aalseth et al. (IGEX Collaboration), Phys. of Atomic Nuclei 63 (2000) 1225. [7] D. Budj´asˇ, M. Barnab´e Heider, O. Chkvorets, N. Khanbekov and S. Sch¨onert, JINST 4 P10007 (2009), doi:10.1088/17480221/4/10/P10007. [8] S.R. Elliott et al., Nucl. Instrum. Meth. A 558 (2006) 504. I. Abt, A. Caldwell, K. Kr¨oninger, J. Liu, X. Liu, B. Majorovits, Nucl. Instrum. Meth. A 583 (2007) 332-340. [9] M. Di Marco, P. Peiffer, S. Sch¨onert, Nucl. Phys. Proc. Suppl. 172 (2007) 45-48.
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